An eye pattern is graphical displaying of overlaid a large number of sampled transitions of a signal waveform. Specifically, as depicted in FIG. 25, an eye pattern is waveforms which are sampled from a transient waveform and which are overlaid on one another, and is used for determination on capability of high-speed transmission. FIG. 25 depicts an example of a typical eye pattern.
A manner of drawing an eye pattern will now be described with reference to accompanying drawings 26A-26C. FIG. 26A depicts an example of a transient waveform; FIG. 26B depicts waveforms sampled in units of a frame from the transient waveform of FIG. 26A; and FIG. 26C depicts an eye pattern obtained by overlaying the waveforms sampled in units of a frame with one another.
In order to draw an eye pattern of a transient waveform as depicted in FIG. 26A, a number of frames of the transient waveform are defined such that each frame has the same width in the forward and the reverse directions from the middle point of the frame, which corresponds to a trigger of the clock period. FIG. 26A depicts four frames 1-4. Each frame region is set to be slightly wider than the signal width for one bit, so that contiguous two frames partially overlap with each other.
The waveform of each frame defined as the above is sampled from the transient waveform depicted in FIG. 26A. FIG. 26B depicts examples of waveforms sampled from such frames 1-4.
Then the waveforms sampled in units of a frame are overlaid on the basis of the timing of the trigger as illustrated in FIG. 26B, and thereby an eye pattern of FIG. 26C is drawn.
Referring to the eye pattern drawn in the above manner, the quality of a waveform can be determined and confirmed in the following manner.
If a number of waveforms in the eye pattern are overlaying with one another at the same point (i.e., the same timing), the width of the waveforms constituting an eye pattern is thin and the center (inner shape) of the eye pattern is widely open. This state is called “the eye is open”. If the eye of an eye pattern of waveform to be determined is open, the waveform to be determined is determined to be good in quality.
Conversely, if the positions (timings) of a number of waveforms differ from one another, the width of the waveforms constituting an eye pattern is thick and the center (inner shape) of the eye pattern is closed. This state is called “the eye is closed”. If the eye of an eye pattern of waveform to be determined is closed, the object circuit is incapable of signal transmission and the waveform to be determined is determined to be poor in quality.
Generally, a requisite opening degree of an eye (inside of an eye pattern) is defined for each standard of a transmission signal to be determined. If the eye of an eye pattern obtained on an object circuit to be determined is below the defined size, the object circuit is not certificated to be satisfy the standard. Examples of a standard of a transmission signal are Universal Serial Bus (USB), Peripheral Component Interconnect (PCI)-Express, and Double Data Rate (DDR) memory.
Here, a normal procedure of generating an eye pattern through simulation will be detailed along a flow diagram (steps S1 and S2) of FIG. 27. Previously to the simulation, circuit data related to an object circuit and test patterns of an input pulse are stored in a database D1 and a database D2, respectively.
The waveforms output from the object circuit in response to inputting a test pattern stored in the database D2 is obtained through a circuit simulation based on the circuit data stored in the database D1 (step S1). In succession, an eye pattern of the waveform obtained in the circuit simulation is generated in the above manner described with reference to FIGS. 26A-26C, and is then displayed on the monitor (step S2).
Recent transmission speed is increasing to make it important to confirm the quality of a waveform using an eye pattern. In particular, in conformation of waveform quality, the inner shape of an eye pattern, that is the shape of the eye, plays an important roll. For this reason, a demand arises to efficiently obtain the inner shape of an eye pattern through a simulation.
However, the inner shape of an eye pattern fluctuates complexly due to inter-symbol interference depending on a bit pattern of an input signal, transmission loss, and multiple reflection in combination of one another. A test pattern that causes the worst case of the inner shape of an eye pattern differs with a circuit condition of an object circuit. For the above, in order to obtain the precise inner shape of an eye pattern (eye shape), various test patterns are input into an object circuit and waveforms responsive to the input of the test patterns are simulated, which takes a long time.